System and method for reduction of drift in a vision system variable lens

ABSTRACT

This invention provides a vision system that is arranged to compensate for optical drift that can occur in certain variable lens assemblies, including, but not limited to, liquid lens arrangements. The system includes an image sensor operatively connected to a vision system processor, and a variable lens assembly that is controlled (e.g. by the vision processor or another range-determining device) to vary a focal distance thereof. A positive lens assembly is configured to weaken an effect of the variable lens assembly over a predetermined operational range of the object from the positive lens assembly. The variable lens assembly is located adjacent to a front or rear focal point of the positive lens. The variable lens assembly illustratively comprises a liquid lens assembly that can be inherently variable over approximately 20 diopter. In an embodiment, the lens barrel has a C-mount lens base.

RELATED APPLICATION

This application is a continuation of co-pending U.S. patent applicationSer. No. 15/847,868, entitled SYSTEM AND METHOD FOR REDUCTION OF DRIFTIN A VISION SYSTEM VARIABLE LENS, filed Dec. 19, 2017, which is acontinuation-in-part of co-pending U.S. patent application Ser. No.14/271,148, entitled SYSTEM AND METHOD FOR REDUCTION OF DRIFT IN AVISION SYSTEM VARIABLE LENS, filed May 6, 2014, the teachings of each ofwhich applications are expressly incorporated herein by reference.

FIELD OF THE INVENTION

This application relates to cameras used in machine vision and moreparticularly to automatic focusing lens assemblies.

BACKGROUND OF THE INVENTION

Vision systems that perform measurement, inspection, alignment ofobjects and/or decoding of symbology (e.g. bar codes, or more simply“IDs”) are used in a wide range of applications and industries. Thesesystems are based around the use of an image sensor, which acquiresimages (typically grayscale or color, and in one, two or threedimensions) of the subject or object, and processes these acquiredimages using an on-board or interconnected vision system processor. Theprocessor generally includes both processing hardware and non-transitorycomputer-readable program instructions that perform one or more visionsystem processes to generate a desired output based upon the image'sprocessed information. This image information is typically providedwithin an array of image pixels each having various colors and/orintensities. In the example of an ID reader, the user or automatedprocess acquires an image of an object that is believed to contain oneor more IDs. The image is processed to identify ID features, which arethen decoded by a decoding process and/or processor to obtain theinherent information (e.g. alphanumeric data) that is encoded in thepattern of the ID.

Often, a vision system camera includes an internal processor and othercomponents that allow it to act as a standalone unit, providing adesired output data (e.g. decoded symbol information) to a downstreamprocess, such as an inventory tracking computer system or logisticsapplication.

An exemplary lens configuration that can be desirable in certain visionsystem applications is the automatic focusing (auto-focus) assembly. Byway of example, an auto-focus lens can be facilitated by a type of“variable lens” assembly (defined further below), known as a so-calledliquid lens assembly. One form of liquid lens, available from Variopticof France uses two iso-density liquids—oil is an insulator while wateris a conductor. The variation of voltage passed through the lens bysurrounding circuitry leads to a change of curvature of theliquid-liquid interface, which in turn leads to a change of the focallength of the lens. Some significant advantages in the use of a liquidlens are the lens' ruggedness (it is free of mechanical moving parts),its fast response times, its relatively good optical quality, and itslow power consumption and size. The use of a liquid lens can desirablysimplify installation, setup and maintenance of the vision system byeliminating the need to manually touch the lens. Relative to otherauto-focus mechanisms, the liquid lens has extremely fast responsetimes. It is also ideal for applications with reading distances thatchange from object-to-object (surface-to-surface) or during thechangeover from the reading of one object to another object—for examplein scanning a moving conveyor containing differing sized/height objects(such as shipping boxes). In general, the ability to quickly focus “onthe fly” is desirable in many vision system applications.

A recent development in liquid lens technology is available fromOptotune AG of Switzerland. This lens utilizes a movable membranecovering a liquid reservoir to vary its focal distance. A bobbin exertspressure to alter the shape of the membrane and thereby vary the lensfocus. The bobbin is moved by varying the input current within a presetrange. Differing current levels provide differing focal distances forthe liquid lens. This lens advantageously can provide a larger aperture(e.g. 6 to 10 millimeters) than competing designs (e.g. Varioptic ofFrance) and operates faster. However, due to thermal drift and otherfactors, there may be variation in calibration and focus setting duringruntime use, and over time in general. A variety of systems can beprovided to compensate and/or correct for focus variation and otherfactors. However, such compensation routines can require processing time(within the camera's internal processor) that slows the lens' overallresponse time in arriving at a new focus. Likewise, such compensationroutines, (e.g. thermal drift) can be standardized, and not customizedto the lens' intrinsics, rendering them less reliable for the specificdrift conditions that a lens may encounter over time. Note that drift ina liquid lens can be, for example, approximately 0.15 Diopter/° C. (i.e.for certain Varioptic liquid lenses currently in production and/orspecified in commercially available products). Some vision applications,especially when small features at a large distance are to be detected,require a stability in optical power of the imager lens of +/−0.1diopter.

Also it is recognized generally that a control frequency of at leastapproximately 1000 Hz may be required to adequately control the focus ofthe lens and maintain it within desired ranges. This poses a burden tothe vision system's processor, which can be based on a DSP or similararchitecture. That is, vision system tasks would suffer if the DSP werecontinually preoccupied with lens-control tasks. All of thesedisadvantages make drift compensation a challenge in many applications.

SUMMARY OF THE INVENTION

This invention overcomes disadvantages of the prior art by providing avision system that is arranged to compensate for optical drift that canoccur in certain lens assemblies capable of varying optical power,wherein the optical power (and hence, varying focal length/distancewhere focal length=1/optical power) is varied by controlling lens shapeand/or lens refractive index. Such lens assemblies include, but are notlimited to, liquid lens arrangements employing, for example, twoiso-density fluids or a flexible membrane—also generally termed a“variable lens” assembly herein. The system includes an image sensoroperatively connected to a vision system processor, and a variable lensassembly that is controlled (e.g. by the vision processor or anotherrange-determining device) to vary a focal distance thereof. A positivelens assembly is configured to weaken an effect of the variable lensassembly over a predetermined operational range of the object from thepositive lens assembly. The variable lens assembly illustrativelycomprises a liquid lens assembly, and such a liquid lens assembly can beinherently variable over approximately 20 diopter. Illustratively, thepositive lens assembly and the variable lens assembly are collectivelyhoused in a removable lens barrel with respect to a camera body and theimage sensor. The image sensor is illustratively located within thecamera body. Likewise, the vision processor can be all, or in part,located in the camera body. In an embodiment, the lens barrel has aC-mount lens base, and the positive lens assembly comprises a doublet,which includes a front convex lens and rear concave lens. The positivelens assembly can define an effective focal range of 40 millimeters.Illustratively, the usable focal length of the lens (e.g. a doublet) isbetween approximately 10 and 100 millimeters. Additionally, the variablelens assembly (e.g. liquid lens assembly) is typically located adjacentto, but remote from, a focal point of the positive lens assembly, whichcan be the front, or more typically, the back/rear focal point of thepositive lens assembly. The distance between the variable lens assemblyand the focal point can be between approximately 0.1 and 0.5 times afocal length F of the positive lens assembly. In this manner, thepositive lens assembly and the variable lens assembly are part of anoverall lens assembly focusing light on the image sensor. The opticalpower of the positive lens assembly, thus, “predominantly defines” anoverall optical power of the overall lens assembly—in other words, themajority of magnification/optical power is provided by the positive lensassembly, thereby minimizing the effect of drift in the variable lensassembly.

In an illustrative embodiment a vision system that compensates for driftis provided. The vision system includes an image sensor operativelyconnected to a vision system processor, a variable lens assembly thatvaries a shape or a refractive index thereof, and a fixed lens assemblyconfigured to weaken an effect of the variable lens assembly over apredetermined operational range of the object. Illustratively, thevariable lens assembly comprises a liquid lens assembly. It can bepositioned between the image sensor and the fixed lens assembly and canbe variable over approximately 20 diopter. Additionally, the fixed lensassembly can define a positive optical power. Illustratively, the fixedlens assembly and the variable lens assembly are housed in a removablelens barrel with respect to a camera assembly body and the image sensor,the image sensor can be located within the camera assembly body. Thecamera assembly body can be electrically connected to the variable lensassembly to provide at least one of power and control thereof, by atleast one of contact pads and a cable assembly. The fixed lens assemblycan comprise one of: (a) a front lens with a front concave surface and arear convex surface and a central biconvex lens spaced from the frontlens, (b) a front biconvex lens and a rear stacked lens assembly with afront positive lens, center biconcave lens and rear positive lens, (c) afront planoconcave lens and a negative lens, a central stacked lensassembly with a biconvex lens and a planoconvex lens, and a rearbiconvex lens and positive lens, (d) a front planoconvex lens andpositive lens and a rear positive lens and negative lens, and (e) afront stacked lens assembly with a biconvex lens and biconcave lens anda rear planoconvex lens and negative lens. Also, at least one lens ofthe fixed lens assembly can comprise a polymer material. By way ofexample, the fixed lens assembly can define an effective usable focalrange of between approximately 0.3 to 8 meters. Also by way of example,the variable lens assembly can be located adjacent to a focal point ofthe fixed lens assembly. The focal point is one of either a front focalpoint or a back focal point of the fixed lens assembly. In embodiments,the fixed lens assembly can comprise a front lens assembly and a rearlens assembly with the variable lens assembly positioned therebetween,in which the rear lens assembly can define a positive optical power.Also in such embodiments, the front lens assembly can have a pair oflenses, each having convex front surfaces and concave rear surfaces anda lens having opposing concave surfaces, and the rear lens assembly canhave a lens having opposing convex surfaces. Illustratively, the fixedlens assembly and the variable lens assembly are part of an overall lensassembly focusing light on the image sensor, in which an optical powerof the fixed lens assembly predominantly defines an overall opticalpower of the overall lens assembly.

In another illustrative embodiment, a variable lens system for a visionsystem having an image sensor that transmits image data to a processoris provided. The system includes a variable lens assembly (such as aliquid lens assembly. The system includes a fixed lens assembly having afocal point. The variable lens assembly is located adjacent to the focalpoint. The fixed lens assembly and the variable lens assembly can bepart of an overall lens assembly focusing light on the image sensor. Theoptical power of the positive lens assembly can predominantly define theoverall optical power of the overall lens assembly. Illustratively, theliquid lens assembly is variable over approximately 20 diopter. Inembodiments, the fixed lens and the variable lens assembly are housed ina removable lens barrel with respect to a camera assembly body and theimage sensor. The image sensor is located within the camera assemblybody. The camera assembly body can be electrically connected to thevariable lens assembly, to provide at least one of power and controlthereof, by at least one of contact pads and a cable assembly.Illustratively, the lens system can comprise one of: (a) a front lenswith a front concave surface and a rear convex surface and a centralbiconvex lens spaced from the front lens, (b) a front biconvex lens anda rear stacked lens assembly with a front positive lens, centerbiconcave lens and rear positive lens, (c) a front planoconcave lens anda negative lens, a central stacked lens assembly with a biconvex lensand a planoconvex lens, and a rear biconvex lens and positive lens, (d)a front planoconvex lens and positive lens and a rear positive lens andnegative lens, and (e) a front stacked lens assembly with a biconvexlens and biconcave lens and a rear planoconvex lens and negative lens.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention description below refers to the accompanying drawings, ofwhich:

FIG. 1 is a diagram of an illustrative vision system arrangement havinga vision system camera with associated vision processor, with a lensassembly that compensates for inherent drift over time, shown acquiringimages of an exemplary object in a scene according to an illustrativeembodiment;

FIG. 2 is a diagram of the ray trace for an exemplary lens system thatincludes a variable lens assembly imaging an object;

FIG. 3 is a diagram of the ray trace for an illustrative lens systemincluding a variable lens assembly and a positive lens assembly ispositioned along the optical axis at a predetermined distance from thevariable lens assembly, to thereby provide a drift-tolerant lens system;

FIG. 4 is a side cross section of a lens unit including a variable lensassembly and positive lens exhibiting drift tolerance according to anillustrative embodiment, showing relative dimensions of the lens barreland components associated therewith;

FIG. 4A is a side cross section of the lens unit of FIG. 4, showing therelative placement of components along the optical axis;

FIG. 5 is a diagram of the ray trace for the illustrative lens unit ofFIG. 4, shown imaging an object at a first distance;

FIG. 6 is a diagram of the ray trace for the illustrative lens unit ofFIG. 4, shown imaging an object at a second distance, longer than thefirst distance;

FIG. 7 is a diagram of the ray trace for the illustrative lens unit ofFIG. 4, shown imaging an object at a first distance;

FIG. 8 is a diagram of the relationship between the positive lensassembly, variable lens assembly and positive lens focal point accordingto embodiments herein;

FIG. 9 is a diagram of an arrangement of lenses for a drift-tolerantlens system in which a variable lens assembly is located between theoptics and the image sensor according to an embodiment;

FIG. 10 is a diagram of an arrangement of lenses for a drift-tolerantlens system in which a variable lens assembly is located between twogroups of optics, placed ahead of an image sensor according to anembodiment;

FIG. 11 is a diagram of an arrangement of lenses for a 12-millimeter,drift-tolerant lens system in which a variable lens assembly is locatedbetween the optics and the image sensor according to another embodiment;

FIG. 12 is a perspective view of a lens assembly that includes the lensarrangement of FIG. 11;

FIG. 13 is a side cross section of the lens taken along line 13-13 ofFIG. 12;

FIG. 14 is a diagram of an arrangement of lenses for a 16-millimeter,drift-tolerant lens system in which a variable lens assembly is locatedbetween the optics and the image sensor according to another embodiment;

FIG. 14A is a diagram of the image circles a rectangular image sensorthat can be employed in the vision system according to an embodiment;

FIG. 15 is a diagram of an arrangement of lenses for a 25-millimeter,drift-tolerant lens system in which a variable lens assembly is locatedbetween the optics and the image sensor according to another embodiment;

FIG. 16 is a diagram of an arrangement of lenses for a 35-millimeter,drift-tolerant lens system in which a variable lens assembly is locatedbetween the optics and the image sensor according to another embodiment;

FIG. 17 is a is a perspective view of a lens assembly that includes aversion of the lens arrangement of FIG. 16; and

FIG. 18 is a side cross section of the lens taken along line 18-18 ofFIG. 17.

DETAILED DESCRIPTION I. System Overview

FIG. 1 details a vision system 100 that includes a vision system cameraassembly 110 and associated lens unit/assembly 120. The construction ofthe lens unit 120 is described further below. In an embodiment, the lensunit 120 is fixed to the camera, or can be removable using a custom orconventional mount base, such as the well-known Cine or “C-mount”. Thecamera includes a body/housing that can house a plurality of operationalcomponents including an image sensor or imager 130 (shown in phantom).In this embodiment, the imager 130 is operatively connected with anon-board vision processor 140 that operates a variety of hardware and/orsoftware processes, generally termed a vision process 142. The visionprocess 142 can include a plurality of software applications that areadapted to perform general purpose or specialized vision system tasks,for example, ID (code) finding and decoding tasks, edge detection, blobanalysis, surface inspection, robot manipulation and/or otheroperations. See, for example exemplary ID 144. The processes 142 caninclude various image acquisition and image manipulation applications aswell—which place image data into a form more appropriate for use invision system tasks—e.g. histogramming, thresholding, etc. These tasksand processes are known to those of skill in the art and can be sourcedfrom a commercial vision system supplier—such as Cognex Corporation ofNatick, Mass. As shown, the illustrative vision system processor 140 iscontained within the camera body. Vision system data in “raw”,pre-processed (e.g. found, undecoded ID image data), or fully processed(e.g. decoded ID data) form can be provided over a wired and/or wirelesslink 144 to an appropriate data-handling system or processor, such as astandalone PC or server system. Alternate systems, such as mobilecomputing devices, cloud-based devices, and the like can be provided inalternate implementations. The data-handling system stores andmanipulates the image-based data as desired by the user—e.g. quality orinventory control. In alternate embodiments, some or all of the visionsystem processors/processes can be instantiated and/or performed in aremote processor (e.g. the computing device/processor 150) that isinterconnected to the camera 110 by an appropriate wired and/or wirelesslink (e.g. link 144) in a manner known to those of skill in the art.

Note, as used herein, the terms “process” and/or “processor” should betaken broadly to include a variety of electronic hardware and/orsoftware based functions and components. Moreover, a depicted process orprocessor can be combined with other processes and/or processors ordivided into various sub-processes or processors. Such sub-processesand/or sub-processors can be variously combined according to embodimentsherein. Likewise, it is expressly contemplated that any function,process and/or processor herein can be implemented using electronichardware, software consisting of a non-transitory computer-readablemedium of program instructions, or a combination of hardware andsoftware. In a system arrangement, such processes/process functions canbe termed as occurring/existing in a corresponding “module” or“element”. For example, an “ID-reading module”, which performs thefunctions associated with reading and/or decoding of ID codes.

The lens assembly 120 is shown aligned along the optical axis OA (withthe plane of the sensor 130) typically arranged perpendicularly to theaxis. The lens assembly 120 and sensor 130 image an object O. The objectO, by way of example, can be any two-dimensional (2D) orthree-dimensional (3D) surface or shape that partially or fully fitswithin the field of view (FOV). In the depicted example, range/distance(do) of the object O from the camera 110 (e.g. from the focal plane ofthe sensor 130) can be varied, but defines a predetermined operatingrange (according to an illustrative embodiment) within which to imagethe object O.

Illustratively, this embodiment compensates for potential optical driftover time in a variable lens (e.g. a liquid lens) that is part of theoverall lens assembly 120 by defining an operating range for the visionsystem at which the influence of the optical power of the variable lenson the optical power of the overall lens assembly (including any fixedlenses therein) is reduced. In this manner, drift is a small componentof the overall focal performance of the lens assembly. This illustrativearrangement provides benefits where the adjustable focus range can bereduced. Thus, this system is useful in various embodiments—such asthose where the distance (do) of the object surface from the focal planeis relatively constant, or this distance (do) varies over a smallrelative distance. Illustratively, the system can be employed in visionsystem applications that read at larger distances, wherein the requiredoptical range is only a small fraction (approximately 2 diopter) of thespecified range of commercially available liquid lenses (20 diopter). Asdescribed above, the variable lens assembly of the embodimentscontemplated herein can include a variety of lens types that are capableof varying optical power. More particularly, in embodiments, the opticalpower (and hence, varying focal length/distance where focallength=1/optical power) is varied by controlling the lens shape and/orthe lens refractive index. Such variable lens assemblies include, butare not limited to, liquid lenses, and a variety of liquid lens typescan be employed including iso-density fluid types (Varioptic), membranetypes (Optotune), etc. Likewise, variable lenses that operate usingother mechanisms, such as electro-mechanical actuation, can be employed.

II. Drift-Reduction Lens Arrangement

By way of further illustration of the concepts of an embodiment, FIG. 2depicts a ray trace diagram of a basic optics arrangement for anexemplary vision system 200 with an exemplary object O1, image sensor230 and generalized variable lens (e.g. a liquid lens (LL1)). The objectO1 is positioned at a distance dl from the variable lens LL1. Thissystem is free of additional lenses and the rays 240 reflected from theobject O1 pass through the variable lens LL1 and are focused directly onthe image sensor 130 as shown. Thus, any minor variation (for example,from drift) in the focus of the variable lens LL1 results in apotentially significant out-of-focus condition that can affect theability of the vision system to render a proper result.

To address such sensitivity to drift and other focal variations in e.g.a liquid lens, reference is now made to FIG. 3, which shows ageneralized optical arrangement for a vision system 300 according to anembodiment. A fixed (non-variable) positive lens PL is located at apredetermined distance d in front of the variable (e.g. liquid) lensassembly LL2, along the optical path between the system and imagedobject O2.

Thus, the optical power A of this system 300 (where A1 is the opticalpower of the positive lens assembly PL, A2 is the optical power of thevariable lens assembly LL2 and d is the distance between the positivelens PL and the variable lens LL2) is:

A=A1+A2−d*A1*A2

If the distance between the variable lens LL2 and the positive lens PLis relatively large, (e.g. d=k/A1 (where k=0.5 . . . 0.9, and representsthe product of the power of the positive lens A1 and distance d; i.e.k=d*A1)), then the overall optical power A of the above-defined systemof lenses with powers A1 and A2, and relative distance d can be writtenas:

A=A1+(1−k)*A2

and the drift, represented as a differential of lens optical power (dA)per unit temperature (dT) (dA/dT) of the system is:

dA/dT=dA1/dT+(1−k)*dA2/dT

meaning that the drift of the over system dA/dT equals the sum of thedrift of the positive lens dA1/dT and (1−k) times the drift of thevariable lens dA2/dT.

In an embodiment, the fixed positive lens PL can be chosen as a glasslens with inherently low drift (i.e. dA1/dT≈0), so compared to theoriginal setup in FIG. 2, it follows that the overall drift dA/dT of thesystem of FIG. 3 is effectively reduced by a factor 1−k (=0.1× . . .0.5×) using the positive lens PL, and the larger power the positive lens(i.e. larger k), the greater the drift reduction in the variable lens.

Reference is now made to FIG. 4, which details a cross section of anintegrated lens unit/assembly 120 for use in the illustrative visionsystem camera 110 of FIG. 1. This lens assembly 120 can include variouselectrical connections and/or leads (shown schematically in phantom ascable 410 and connector 412) that extend from the variable (e.g. liquid)lens assembly 420 to a location on the body of the camera 110 incommunication with appropriate control processors/components that areassociated with the vision processor 140. Note that the exemplary liquidlens assembly 420, which can be a membrane-type, iso-density fluid-typeor equivalent, is contained within a barrel 430 and the lead 410 isconstructed to extend from a location on the barrel 430. This connectionallows control signals to power the liquid lens assembly (e.g. currentand/or voltage modulation) to enable the variation and setting of thefocus of the liquid lens assembly 420 in response to commands of theprocessor. Proper focus can be determined and/or set using a variety oftechniques known to those of skill—for example using the crispness ofimaged edges after stepping through various focus settings and/or usingan external range-finding device. While the use of a separate cablelink, with associated connector on the body of the camera is employed inthe depicted embodiment, the connection arrangement can be internal tothe barrel 430—for example consisting of aligned contact pads and/orcontact rings (on the lens and camera body) that interconnect when thelens assembly 120 is secured to the camera body.

The lens assembly barrel 430 is sized and arranged in this embodimentwith the form factor of a conventional C-mount lens, having anappropriately threaded base 440. The depicted external thread of thebarrel base (flange) 440 is adapted to mate with a correspondinginternal thread (not shown) on the camera body. The thread size isconventional (e.g. 1 inch×32). Note that the camera body can include avariety of accessories and functional components, such as a ringilluminator surrounding the lens and/or connections for an externalillumination assembly. Such accessories and/or components can be appliedto the camera to accomplish specific vision system tasks. The barrel 430can be constructed from a variety of materials such as cast or machinedaluminum alloy. The threaded base allows the barrel, and associatedoverall lens assembly contained therein, to be removably attached to thecamera body and replaced with other types of lenses at the option ofeither the manufacturer or user. While the form factor of a C-mount baseis used in this embodiment, any acceptable lens base form that allowsaccommodation of a liquid lens or other appropriate variable lens can beemployed in alternate embodiments. For example, an F-mount lens base canbe employed.

The dimensions of the lens barrel 430 are shown by way of non-limitingexample in FIG. 4. As depicted, the barrel outer diameter ODL can beapproximately 28-29 millimeters in an embodiment. This addresses thegeneral size constraints/parameters of a C-mount lens. Likewise, thelength OLL of the barrel 430 from front end 432 to the threaded base 440is illustratively, approximately 32-34 millimeters. The distance DS fromthe lens base 440 to the focal plane of the image sensor 130 isapproximately 17.5 millimeters. Note that these dimensions areillustrative of a wide range of possible relationships that are known tothose of skill.

With further reference to FIG. 4A, the positioning of the internaloptical components of the lens is described in detail. A positive lensassembly 450, having a relatively large diameter with respect to thevariable lens (420) diameter, is located adjacent to the front end 432of the barrel 430. This positive lens assembly (also termed the“positive lens”) 450 is seated within a recess 454 formed at the frontend of the barrel. The positive lens 450 is secured at its front side bya threaded ring 456. Note that this arrangement is highly variable inalternate embodiments, and a variety of mounting and/or attachmentmechanisms can be employed in alternate embodiments. The positive lens450 is an achromatic doublet, defining an effective focal length (f) of40 mm and a back focal length of 33.26 millimeters. The clear apertureis 24 millimeters. The overall lens assembly diameter is 25 millimeters.Illustratively, it consists of a front, convex lens 458 and a rear,concave lens 459. The convex lens 458 defines a front radius RL1 of27.97 millimeters and rear radius RL2 of −18.85 millimeters (wherepositive and negative radii represent directions with respect to theorientation of the imaged object, with positive radii oriented towardthe object and negative radii oriented toward the image sensor). Theconcave lens 459 defines a front radius (also RL2) of 18.85 millimeters(complimenting the mating surface of the convex lens 458) and a rearradius RL3 of 152.94 millimeters. The convex lens 458 has a centerthickness TC1 (along the optical axis OA) of 9.5 millimeters and theconcave lens has a center thickness TC2 of 2.5 millimeters. Thesedimensions are highly variable in alternate embodiments. Theabove-described embodiment and associated dimensions of a positive lens(e.g. doublet) assembly 450 is commercially available from Edmund OpticsInc. of Barrington, N.J. as stock number 32321. In this embodiment, thelens front-to-sensor plane distance ODLF is approximately 49 millimetersaccording to an embodiment. It should be clear that the positive lens'dimensions and/or the arrangement of components are highly variable inalternate embodiments.

The variable (e.g. liquid) lens assembly (which can be sourced from avariety of manufacturers) 420 is positioned adjacent to the rear end ofthe lens barrel 430. In this embodiment, and by way of non-limitingexample, the variable lens assembly 420 can comprise a model Arctic 416liquid lens available from Varioptic of France. The exemplary variablelens assembly has a focus range of approximately 20 diopter (i.e. 5centimeters to infinity), a diameter of 7.75 millimeters and a thickness(along the optical axis) of 1.6 millimeters. The depicted, exemplary,liquid lens assembly 420 consists of the lens unit 470, which is mountedon a controller circuit board 472, having a central aperture 474,aligned along the optical axis through which focused light passes ontothe sensor 130.

The lens assembly 130 can be supported within the barrel 430 using anintegral or unitary spacer, shoulder arrangement and/or supportstructure 460. The support structure 460 ensures that the variable lensassembly 420 remains fixed in an appropriate alignment with respect tothe optical axis OA. The distance DLR from the positive lens rear to thefront of the variable lens unit 470 is 18.0 millimeters in thisembodiment. Note that the image sensor 130 can define a conventional ½inch-size CMOS sensor (6.9 millimeters (horizontal) by 5.5 millimeters(vertical)—SW in FIG. 5) in an embodiment.

Reference is now made to FIGS. 5-7, which show the vision system andlens assembly in operation at a plurality of focal distances within theoperational range of the system. The object O is thus located at threeexemplary distances DO1, DO2 and DO3 in each of ray trace diagrams ofFIGS. 5, 6 and 7, respectively. By way of example, DO1 is approximately219 millimeters, DO2 is approximately 430 millimeters, and DO3 isapproximately 635 millimeters. Within this range, the optical power ofthe variable lens assembly is varied from +10.73 diopter for F=37.4millimeters (FIG. 5); to +0.32 diopter for F=39.8 millimeters (FIG. 6);to −3.81 diopter for F=42.3 millimeters. This 219 to 635-millimeterfocal range is associated with a 6.9 diopter variation. By way ofcomparison, a system mounting with the depicted variable lens assemblyin a conventional arrangement with the variable lens attached at closedistance of the front lens typically requires a 3.3 diopter variation.Thus, the illustrative system effectively reduces potential drift bymore than a factor of 2 relative to a conventional arrangement.

More generally, the variable lens assembly (e.g. liquid lens assembly)is located adjacent to, but remote from, a focal point of the positivelens assembly, which can be the front, or more typically, the back/rearfocal point of the positive lens assembly. It is understood that thepositioning adjacent to the focal point allows for the variable lens tocontribute to the total power of the lens system. The distance betweenthe variable lens assembly and the focal point can be betweenapproximately 0.1 and 0.5 times a focal length F of the positive lensassembly. By way of illustration, reference is made to the diagram ofFIG. 8, where a positive lens assembly PL is positioned along theoptical axis OA with a variable lens VL adjacent to the positive lensfocal point FP. The focal length F between the positive lens PL andfocal point FP is depicted. The distance (1−k)*F is characterized as thedistance between the variable lens VL and the focal lens and the focalpoint FP with k=0.9 to 0.5 (i.e. 0.9*F to 0.5*F). Thus, the distancebetween the positive lens PL and the variable lens VL is k*F (i.e. 0.1*Fto 0.5*F). In this manner, the positive lens assembly PL and thevariable lens VL assembly are part of an overall lens assembly LAfocusing light on the image sensor, and the optical power of thepositive lens assembly “predominantly defines” an overall optical powerof the overall lens assembly—in other words, the majority ofmagnification/optical power is provided by the positive lens assembly,thereby minimizing the effect of drift in the variable lens assembly.

Reference is now made to FIGS. 9 and 10, which show two embodiments ofdrift-reducing lens arrangements according to embodiments. The tableshereinbelow also provide, respectively, exemplary parameters for eachlens element. FIG. 9 is an arrangement 900 of lenses in association withan image sensor 910 of (e.g.) conventional design. In this embodiment,variable lens comprises a liquid lens assembly 920. The depicted rays930 are shown reflected into the lens arrangement 900 from an object(not shown) that can be placed at a distance of (e.g.) 200 millimetersfrom the first lens 940. By way of non-limiting example, this lens 940comprises a front concave surface 942 and a rear convex surface 944.This lens can comprise a polymer, such as polycarbonate (or anotheroptically suitable material). A central lens assembly includes a frontcomposite lens 950 with a convex lens 952 having a front surface 954 anda rear surface 956. This mates with a concave lens 958 with convexsurface of similar radius and a rear convex surface 960. Note that thelens element(s) (950) can also be constructed from polycarbonate (oranother optical material). A disk-shaped optical element 970 (e.g. an IRfilter) with infinite radii on each side (e.g. parallel planes) islocated behind the composite lens assembly 950. The rays 930 convergefrom the disk 970 at the variable (liquid) lens assembly 920. Thisexemplary assembly can be based around the Arctic 416 lens fromVarioptic of France, or another appropriate (e.g.) liquid lens. Itcomprises a front cover disk 980, lens element 982, interconnected withthe lens control circuitry 990, aperture stop (with associated radius of341.763 millimeters) 984 and rear cover disk 986. The lens controlcircuitry can be operatively connected with the vision system describedabove. This element is adjusted to maintain focus on the image sensor910 and resists drift based on a variety of conditions described above.The spacing between the liquid lens assembly 920 and imager can beapproximately 13-14 millimeters and the spacing between the disk 970 andliquid lens assembly 920 can be approximately 3-4 millimeters. Note thatthe size and shape of the exemplary lenses can be modified in accordancewith skill in the art, as well as the spacing therebetween. Likewise,the variable lens assembly can comprise a variety of different types,operating on differing physical principles. For example a membrane-typeliquid lens available from Optotune of Switzerland can be substituted,as well as mechanical lens types.

Referring now to FIG. 10, another embodiment of a lens arrangement 1000capable of low-drift over a given working range, is depicted. Thisarrangement includes an image sensor 1010 of conventional design and avariable (liquid) lens assembly 1020. Rays 1030 reflect from an object(not shown) at a range of (e.g.) 80-100 millimeters to a front convexlens 1040. By way of non-limiting example, the front lens 1040 includesa front convex surface 1042 and a rear concave surface 1044. The nextlens 1046 defines a front convex surface 1048 and a rear concave surface1050. The next lens 1060 is concave on both surfaces 1062 and 1064, witha front surface 1062 and a rear surface 1064. Rays 1030 exit this lens1060 and are directed into the variable (e.g. liquid) lens assembly1020, which, in this embodiment, is in the middle of the opticalarrangement, with additional lenses 1080 and 1088 positioned between itand the image sensor 1010. In this example, the liquid lens assembly1020 is similar in model and construction to the assembly 920 describedabove (with an aperture stop having a radius of 10.101 millimeters), andis controlled by a lens controller 1090 that can operate similarly tothe controller 990 also described above. Alternate embodiments of thevariable lens assembly can be employed where appropriate, as describedwith reference to the arrangement 900 above. Rays 1030 from the variable(e.g. liquid) lens assembly 1020 are directed into a convex lens 1080spaced at approximately 1.2 millimeters from the liquid lens assembly.The convex lens 1080 includes a front convex surface 1082 and a rearconvex surface 1084. A disk-shaped (e.g.) IR filter 1088 can be locatedbehind the convex lens 1080. It is spaced approximately 10-12millimeters in front of the image sensor in this embodiment. The lensesare, by way of non-limiting example, constructed from optical glass inthis embodiment, but one or more of the lenses (or other opticalelements) can be constructed from another acceptable material, such aspolycarbonate, or an appropriate equivalent material.

The lens arrangements 900 and 1000 described above can be adapted to beenclosed in a lens package with (e.g. a conventional camera base mount,such as a C-mount, as described above). Appropriate electricalconnectors can be provided between the lens body and the camera base toenable control of the variable lens assembly. The electronics of thecontrol circuit can reside in whole or in part with respect to the lensbody, or within the camera body as appropriate.

By way of non-limiting example, the lenses of the various embodimentsherein can define the specified parameters in each of the tablespresented below. The parameters for the lens assembly 900 (FIG. 9) areprovided in the first table hereinbelow, with the associated front andrear surfaces (as applicable) of each structure or element in theoverall assembly ordered (left-to-right) respectively from 0-13:

Thickness or Semi- Distance to diam- Sur- Ref. Radius Next Surface eterface Structure # (mm) (mm) Material (mm) 0 Object 200.414 (not shown) 1lens 940 942 −5.758 3.543 Polycarbonate 3.28 2 944 −6.977 4.121 4.00 3lens 950 954 11.027 3.012 480R + PC 4.00 4 956 −5.976 0.792 4.00 5 960−48.925 0.100 4.00 6 filter 970 infinty 1.300 B270 4.00 (flat) 7infinity 3.000 4.00 (flat) 8 liquid lens infinity 0.550 multiple 2.00920 (flat) 9 infinity 2.00 (flat) 10 variable 1.15 11 infinity 0.3002.00 (flat) 12 infinity 13.884 2.00 (flat) 13 Image 910The table below is for the lens assembly 1000 (FIG. 10), with theassociated front and rear surfaces (as applicable) of each structure orelement in the overall assembly ordered (left-to-right) respectivelyfrom 0-14:

Thickness or Semi- Distance to diam- Ref. Radius Next Surface eterSurface Structure # (mm) (mm) Material (mm) 0 Object 82.081 (not shown)1 lens 1040 1042 13.078 1.461 N-LASF9 4.00 2 1044 49.620 0.256 4.00 3lens 1046 1048 19.922 1.770 N-SF6 4.00 4 1050 18.444 1.066 4.00 5 lens1060 1062 −25.000 1.495 N-SF6 4.00 6 1064 25.000 2.000 4.00 7 liquidlens infinity 0.550 multiple 2.00 1020 (flat) 8 infinity 2.00 (flat) 9variable 1.15 10 infinity 0.300 2.00 (flat) 11 infinity 1.200 2.00(flat) 12 lens 1080 1082 23.854 1.400 N-LASF9 3.00 13 1084 −18.000 1.4333.00 14 filter 1088 infinity 1.300 B270 3.00 (flat) 15 infinity 11.1003.00 (flat) 16 Image 1010

It is further contemplated that the drift-compensating lens arrangementof the embodiments herein can be employed in combination with otherdrift-reducing methods, such as temperature stabilization of thevariable lens or optical feedback systems. By way of non-limitingexample, and incorporated herein by reference as useful backgroundinformation, such arrangements are shown and described in commonlyassigned U.S. Pat. No. 8,576,390, entitled SYSTEM AND METHOD FORDETERMINING AND CONTROLLING FOCAL DISTANCE IN A VISION SYSTEM CAMERA, byNunnink. Reference is also made to U.S. patent application Ser. No.14/139,867, entitled CONSTANT MAGNIFICATION LENS FOR VISION SYSTEMCAMERA, by Nunnink; U.S. patent application Ser. No. 13/800,055,entitled LENS ASSEMBLY WITH INTEGRATED FEEDBACK LOOP FOR FOCUSADJUSTMENT, by Nunnink et al. Illustratively, this application providesa removably mountable lens assembly for a vision system camera thatincludes an integral auto-focusing, liquid lens unit, in which the lensunit compensates for focus variations by employing a feedback controlcircuit that is integrated into the body of the lens assembly. Thefeedback control circuit receives motion information related to andactuator, such as a bobbin (which variably biases the membrane undercurrent control) of the lens from a position sensor (e.g., a Hallsensor) and uses this information internally to correct for motionvariations that deviate from the lens setting position at a target lensfocal distance setting. The position sensor can be a single unit, or acombination of discrete sensors located variously with respect to theactuator/bobbin to measure movement at various locations around the lensunit. Illustratively, the feedback circuit can be interconnected withone or more temperature sensors that adjust the lens setting positionfor a particular temperature value. In addition, the feedback circuitcan communicate with an accelerometer that senses the acting directionof gravity, and thereby corrects for potential sag (or otherorientation-induced deformation) in the lens membrane based upon thespatial orientation of the lens.

III. Drift-Reduction Lens Assembly

FIGS. 11-18 variously describe embodiments of a drift-reduction lensallowing for extended range reading of object features (e.g. ID codes)for use in a variety of camera assemblies and associated applications,including handheld and fixed-mount units. Focal ranges of up toapproximately 8 meters can be imaged using the lens arrangements herein.In general, the illustrative arrangements provide a variable-focus (e.g.liquid) lens positioned behind the remaining, fixed lens opticspackage—such that the variable lens is generally at the rear of the lensassembly, and between the fixed optics package and camera image sensor.With reference to FIG. 11, a lens arrangement 1100 is shown. Thisarrangement is applicable to a 12-millimeter (f′=12) lens. As shown, therelative scale 1110 of the overall lens arrangement (in millimeters) isprovided. The lens' fixed optics (shown in dashed box 1120) consists ofa front plate element 1130, followed by a biconvex lens 1132. A set ofthree, smaller diameter lenses 1134 (positive), 1136 (biconcave) and1138 (positive—opposite facing) are provided behind the biconvex lens1132. In this embodiment, the fixed optics package 1120 is provided in aseparate lens housing, while a variable-focus lens assembly 1140 ismounted within the framework of the vision system housing (e.g. ahandheld ID reader, such as described in commonly assigned U.S. patentapplication Ser. No. 14/550,709, entitled IMAGE MODULE INCLUDINGMOUNTING AND DECODER FOR MOBILE DEVICES, filed Nov. 21, 2014). By way ofnon-limiting example, the variable lens assembly 1140 can comprise theabove-described liquid lens mechanism, available from Optotune ofSwitzerland. The variable lens assembly can alternatively comprise anyacceptable, manually or electronically adjustable lens arrangement,including those described above, available from Varioptic of France. Thevariable lens assembly 1140 can be interconnected (via a cable, printedcircuit traces, etc.) to the vision system processor or anothercontroller that allows the focal length of the lens to be adjusted. Thiscontroller can be integrated with the above-described feedback system.The variable lens assembly 1140 is optionally arranged with one or morefilters and/or dust covers as appropriate. An aperture stop 1142 is alsoprovided in this embodiment. The variable lens assembly 1140 focuseslight (rays 1150) onto the image sensor 1152 for transmission to thevision system processor. The overall length 1160 of arrangement 1100between the front surface of the plate 1130 and image sensor 1152 isapproximately 15.2 millimeters. The distance 1162 between the rear faceof the rear lens 1138 and the image plane (image sensor 1152) isapproximately 10.26 millimeters. By way of example, the approximateparameters of the arrangement 1100 define F/# of 7; an image radius of 3millimeters (i.e. ⅓ inch at 1.2-Megapixel sensor, up to a 5.0-Megapixelsensor); an RMS spot radius of 1.7 μm for 3 mm image height; and ameasured distortion of less than 3%-4%.

The table below is for the lens assembly 1100 (FIG. 11), with theassociated front and rear surfaces (as applicable) of each structure orelement in the overall assembly ordered (left-to-right) respectivelyfrom 0-16:

Thickness or distance to Semi- Radius next surface diameter SurfaceStructure (mm) (mm) Material (mm) 0 Object 500 (not shown) 1 filter 1130infinity 0.650 B-270 2.50 (flat) 2 infinity 0.200 2.50 (flat) 3 lens1132 11.175 1.300 N-SK16 2.50 4 −11.175 0.200 2.50 5 lens 1134 3.7650.850 N-SK16 1.50 6 5.228 0.336 1.00 7 lens 1136 −5.227 0.800 N-SF2 1.508 5.227 0.150 1.20 9 aperture stop 0.150 multiple 0.63 1142 10 lens 1138−8.538 0.800 N-BAF10 1.20 11 −3.331 1.100 1.50 12 liquid lens variablemultiple 1.60 13 1100 infinity 1.60 (flat) 14 sensor window infinity0.400 D263T 1152 (flat) 15 infinity 0.125 (flat) 16 Image 1160

Note that the various tables of lens parameters presented above andfurther below are only by way of example of a wide range of possibleimplementations. It should be clear to those of skill that any or all ofthe lenses and/or optical components herein can be altered by employingdifferent parts, sizes, focal lengths, thicknesses, etc. as appropriateto the mechanical and optical needs of the imaging application.

FIGS. 12 and 13 show a lens assembly 1200 corresponding to the fixedoptics package 1120. The lens elements are contained within a barrelhousing 1210 constructed from aluminum, or another acceptable material.The lens elements are similarly numbered to their counterparts in theassembly 1120 of FIG. 11. The base 1230 of the lens 1200 can be anyacceptable format—for example, a C-mount threaded base (i.e. 1 inch×32threads-per-inch) can be specified for the full length of the barrel.Alternatively, an M8×0.5 thread can be specified for the full length ofthe barrel, or in either case, an appropriate portion thereof. As shownin the cross section of FIG. 13, an aperture stop 1310 can be locatedbetween the biconcave lens 1136 and rearmost positive lens 1138. Thelenses 1130-1138 contained within the barrel 1210 are retained by afront retaining ring 1240 with an outside diameter 1330 of 10millimeters, which is threaded onto the front end of the barrel 1210. Athreaded spacer ring 1250 is also threaded onto the barrel, and islocated therealong so as to set the focal distance of the lens assemblywith respect to the image plane. In an embodiment, when the ring 1250 isproperly located on the lens barrel 1210, it can bepermanently/semi-permanently secured to the barrel using thread-lockingcompound, adhesive or another fixing mechanism (e.g. a set screw, pin,etc.). When the lens is threaded into the device's lens mount, the ring1250 bears against the mounting and provides a desired spacing. In anembodiment, the overall lens length 1340 is approximately 6.9millimeters, and the set distance 1350 between the rear face of theretaining ring 1250 and the image plane 1360 is approximately 12.15millimeters.

FIGS. 14-18 variously depict versions of a drift-reduction lens assemblythat can include the variable lens within its overall structure and thatcan be employed in (e.g.) fixed mount vision systems—for example, IDreaders used in logistics and object tracking applications.

With reference to FIG. 14, a 16-millimeter lens arrangement 1400 isshown. This arrangement can be constructed with a housing 1410 thatincludes the variable (e.g. liquid) lens assembly 1430 within theoverall package. The lens assembly is connected via a cable 1432 orother modality to a connector/contacts on the camera, or other visionsystem housing, which communicates with the processor so that the focaldistance of the lens assembly 1430 can be controlled. Note that avariety of circuitry can be built into/onto the lens housing to performsome or all of the variable-lens-control functions.

The lens arrangement 1400 includes a front negative lens 1440, followedby a smaller-diameter negative lens 1442, another biconvex lens 1444,and a smaller-diameter doublet 1445 consisting of a biconvex lens 1446and planoconcave lens 1448. A second, smaller diameter doublet 1450,consisting of a positive lens 1452 and biconvex lens 1454, is providedbehind the first doublet 1445, and a positive lens 1456 is providedbetween the doublet 1450 and variable (liquid) lens assembly 1430. Anaperture stop 1458 can also be provided at the rear surface of the lastpositive lens 1456. The relative scale 1470, in millimeters, is depictedand the back focal length 1480 between the rear of the variable lens1430 and the image plane on the surface of the image sensor 1490 is setat approximately 8.5 millimeters—using (e.g.) appropriate adjustmentrings, bases, mounts, etc. that should be clear to those of skill. Asshown briefly in FIG. 14A, this generates an image circle ofapproximately 8 millimeters in diameter. This lies within the maximumimage circle 1496 (approximately 8.83 millimeters) of the depicted,exemplary sensor 1490 is an IMX265 model image CMOS sensor (by SonyCorporation of Japan). That is, the image circle 1496 circumscribes thecorners of the rectangular perimeter 1495, which represents the useablearray of image pixels for the sensor 1490. Other image sensors, such asthat having a pixel array defined by the rectangle 1494, arecharacterized by a different (in this example, smaller—e.g. 7.66millimeters) image circle dimension (1493). Such a smaller-dimensionsensor is available from Teledyne e2v, Ltd. (UK).

Other exemplary optical parameters of the lens assembly 1400 can includea focal length of approximately 16.2 to 16.6 millimeters, aperture sizeof F8, a total track of approximately 27.9 millimeters, a focus range of1.0-4.0 meters and a working range for the variable lens of betweenapproximately 0.0 and 2.5 diopters. Within this range, there istheoretically 2.5-times less drift than a conventional design. The RMSspot radius is below 2.2 microns in the extreme field of view (FOV)position.

The table below is for the lens assembly 1400 (FIG. 14), with theassociated front and rear surfaces (as applicable) of each structure orelement in the overall assembly ordered (left-to-right) respectivelyfrom 0-20:

Thickness or Distance to Semi- Radius Next Surface diameter SurfaceStructure (mm) (mm) Material (mm) 0 Object 1828.571 (not shown) 1 lens1440 25.720 1.143 N-BK7 3.886 2 6.657 1.000 3.000 3 lens 1442 14.1302.000 N-SK16 3.000 4 5.789 1.000 2.500 5 lens 1444 20.400 0.914 N-SK162.500 6 −32.069 0.229 2.266 7 doublet 1445 10.082 1.234 N-SK16 + 2.237 8−32.093 1.097 N-SF2 2.129 9 32.093 0.200 1.988 10 doublet 1450 5.8501.097 N-SF2 + 2.000 11 5.000 1.500 N-BK7 2.000 12 −12.411 0.229 2.000 13lens 1456 6.259 1.000 N-SF2 2.000 14 3.130 0.229 0.920 15 aperture stop1.000 multiple 0.898 1458 16 liquid lens variable multiple 1.600 17 1430infinity 9.740 1.600 (flat) 18 sensor window infinity 0.400 D263T 1490(flat) 19 infinity 0.125 (flat) 20 Image 1494

With reference to FIG. 15, a 25-millimeter lens arrangement 1500 isshown. This arrangement can be constructed with a housing 1510 thatincludes the variable (e.g. liquid) lens assembly 1530 within theoverall package. The lens assembly is connected via a cable 1532 orother modality to a connector/contacts as described above. The lensarrangement 1500 includes a front lens 1540 with a slightly concavefront face, followed by a smaller-diameter planoconvex lens 1542, and adoublet 1544 consisting of a biconvex lens 1546 and biconcave lens 1548.A second, smaller-diameter doublet 1550, consisting of a first positivelens 1552 and second lens 1554, is provided behind the first doublet1544, and a negative lens 1556 is provided between the doublet 1550 andvariable (liquid) lens assembly 1530. An aperture stop 1558 can also beprovided at the rear surface of the last positive lens 1556. Therelative scale 1570, in millimeters, is depicted and the back focallength 1580 between the rear of the variable lens 1530 and the imageplane on the surface of the image sensor 1590 (e.g. approximately an8-millimeter image circle) is set at approximately 8.5 millimeters—using(e.g.) appropriate adjustment rings, bases, mounts, etc. that should beclear to those of skill. Other parameters of the lens assembly include afocal length of approximately 24.2 to 25.2 millimeters, aperture size ofF8, a total track of approximately 27.6 millimeters, a focus range of1.0-4.0 meters and a working range for the variable lens of betweenapproximately 0.0 and 4.0 diopters. Within this range, there istheoretically four-times less drift than a conventional design. The RMSspot radius is below 1.9 microns in the extreme field of view (FOV)position.

The table below is for the lens assembly 1500 (FIG. 11), with theassociated front and rear surfaces (as applicable) of each structure orelement in the overall assembly ordered (left-to-right) respectivelyfrom 0-16:

Thickness or Distance to Semi- Radius Next Surface diameter SurfaceStructure (mm) (mm) Material (mm) 0 Object variable (not shown) 1 lens1540 141.523 1.786 N-SK16 6.071 2 infinity 0.300 6.071 (flat) 3 lens1542 24.589 1.429 N-SK16 4.286 4 179.888 0.357 4.286 7 doublet 154411.789 1.929 N-SK16 + 4.286 8 −13.653 1.714 N-SF2 4.286 9 13.653 1.4343.571 10 doublet 1550 6.888 1.714 N-SF2 + 2.500 11 8.747 1.071 N-BK72.500 12 14.691 0.357 1.786 13 lens 1556 8.245 0.714 N-BK7 2.500 144.122 0.357 1.786 15 aperture stop 1.429 multiple 0.850 1558 16 liquidlens variable multiple 1.600 17 1530 infinity 8.500 1.600 (flat) 14sensor window infinity 0.400 D263T 1590 (flat) 15 infinity 0.125 (flat)16 Image 1594

With reference to FIG. 16, a 35-millimeter lens arrangement 1600 isshown. The lens arrangement in this embodiment is adapted for use inlarge-scale camera assemblies, such as those employed in high-volumelogistics operations (e.g. fulfillment services, mass shipping, etc.)involving various-size objects. This lens arrangement 1600 can beconstructed with a housing 1610 (shown in FIG. 16 as a dashed box, anddescribed in further detail below), which includes the variable (e.g.liquid) lens assembly 1630 within the overall package. The lens assemblyis connected via a cable 1632 or other modality to a connector/contactsas described above.

The lens arrangement 1600 includes a front large-diameter planoconvexlens 1640, followed by a smaller-diameter biconvex lens 1642, stackedwith a doublet 1644 consisting of a positive lens 1646 and biconcavelens 1648. A second, smaller-diameter doublet 1650, consisting of afirst positive lens 1652 and second planoconvex lens 1654, is providedbehind the first doublet 1644, and a negative lens 1656 is providedbetween the doublet 1650 and variable (liquid) lens assembly 1630. Anaperture stop 1658 can also be provided on the rear surface of the lastnegative lens 1656. The relative scale 1670, in millimeters, is depictedand the back focal length 1680 between the rear of the variable lens1630 and the image plane on the surface of the image sensor 1690 (e.g.an 8-millimeter image circle) is set at approximately 8.5 millimeters.Other parameters of the lens assembly include a focal length ofapproximately 32.4 to 34.8 millimeters, aperture size of F8, a totaltrack of approximately 49.6 millimeters, a focus range of 1.0-4.0 metersand a working range for the variable lens of between approximately 0.0and 6.5 diopters. Within this range, there is theoretically 6.5-timesless drift than a conventional design. The RMS spot radius is below 3.4microns in the extreme field of view (FOV) position.

The table below is for the lens assembly 1600 (FIG. 16), with theassociated front and rear surfaces (as applicable) of each structure orelement in the overall assembly ordered (left-to-right) respectivelyfrom 0-20:

Thickness or Distance to Semi- Radius Next Surface diameter SurfaceStructure (mm) (mm) Material (mm) 0 Object variable (not shown) 1 lens1640 55.586 2.500 N-SK16 8.500 2 infinity 10.000 8.500 (flat) 3 lens1642 20.856 3.000 N-SK16 6.000 4 −24.197 0.500 6.000 7 doublet 1644−20.861 2.700 N-SK16 + 6.000 8 17.704 2.400 N-SF2 6.000 9 17.704 7.4795.000 10 doublet 1650 8.787 2.400 N-SF2 + 3.500 11 9.034 1.500 N-BK73.500 12 — 0.500 2.500 2195.069 13 lens 1656 11.052 1.000 N-BK7 3.500 145.526 0.500 2.500 15 aperture stop 2.000 0.892 1658 16 liquid lensvariable multiple 1.600 17 1630 infinity 8.500 1.600 (flat) 18 sensorwindow infinity 0.400 D263T 1690 (flat) 19 infinity 0.125 (flat) 20Image 1694The table below is for the lens assembly 1800 (FIG. 18), with theassociated front and rear surfaces (as applicable) of each structure orelement in the overall assembly ordered (left-to-right) respectivelyfrom 0-22:

Thickness or Distance to Semi- Radius Next Surface diameter SurfaceStructure (mm) (mm) Material (mm) 0 Object variable (not shown) 1 filter1840 infinity 2.000 multiple 10.750 (flat) 2 infinity 3.500 10.750(flat) 3 lens 1842 infinity 2.000 N-SK16 9.500 (flat) 4 −46.710 1.0009.500 5 doublet 1844 19.310 2.700 N-SK16 + 7.500 6 infinity 2.400 N-SF27.500 (flat) 7 62.220 1.500 7.000 8 lens 1850 −30.000 2.000 N-SF2 7.5009 30.000 1.000 7.000 10 lens 1852 24.550 3.400 N-SK16 7.500 11 300.0001.000 7.000 12 lens 1854 −300.000 3.400 N-SF2 7.000 13 −24.550 1.0007.500 14 doublet 1856 18.000 2.400 N-SF2 + 4.000 15 9.000 1.500 N-SK164.000 16 12.700 1.000 3.500 17 lens 1858 30.000 1.500 N-SK16 4.000 1817.140 0.500 3.000 19 aperture stop 0.987 1859 20 liquid lens variable1.600 21 1860 infinity 9.103 multiple 1.600 (flat) 22 Image (not shown)

As shown in FIGS. 17 and 18, a further embodiment and/or implementationof the 35-millimeter reduced-drift lens 1700 is shown in further detail.This lens assembly 1700 includes an outer housing 1710 that encloses aseries of lenses that are functionally similar or identical to theabove-described arrangement 1600 in FIG. 16. The housing can beconstructed from any acceptable material (e.g. aluminum alloy), and in avariety of shapes. As shown, the housing 1710 includes a front end 1720,main barrel 1730 and rear end 1740. With further reference to FIG. 18,the lens front 1720 is threaded into an internal thread, which is formedin a widened flange 1820 of the main barrel 1730. Note that an optionalfilter (e.g. a red bandpass filter) 1840 can be fitted to the lens frontin this or other embodiments. The overall diameter DF of the lens frontend 1720 is approximately 27.5 millimeters. In general, the filter 1840can be a commercially available, threaded filter of appropriate opticalspecifications (e.g. wavelength bandpass for visible color, IR, UV,etc.). The main barrel 1730 houses a planoconvex lens 1842 in front of adoublet 1844 consisting of a planoconvex lens 1846 and planoconcave lens1848 that together generate a positive lens geometry. A biconcave lens1850 is stacked behind the doublet 1844. A smaller-diameter pair ofopposing planoconvex lenses 1852 and 1854 is provided behind thebiconcave lens 1850. A smaller-diameter doublet 1856 defines a negativelens behind the lenses 1852 and 1854. This doublet 1856 resides behind arearmost positive lens 1858. The variable (i.e. liquid) lens 1860resides behind the lens 1858. It is retained in the smaller-diameterrear end 1740 by a threaded, annular retaining ring 1862 that sitsinside a rear collar 1864 with (e.g.) an M13×0.5 internal thread. Theinner diameter IDC of the collar is approximately 13 millimeters(threaded) and the outer diameter ODC is approximately 14 millimeters,and its axial length LC can be approximately 3.1 millimeters. Theretaining ring 1862 can include a slot 1866 for in tightening by ablade-shaped tool of appropriate side and shape. Note that the lensarrangement 1700 can also include an aperture stop 1859 within theoptical path at an appropriate location—for example adjacent to theliquid lens assembly 1860 on the rear surface of the lens 1858.

The barrel 1730 can be threaded at the back end 1734 to mate with aninternal thread on a camera assembly lens mount. The depth of mountingis controlled by an adjustment sleeve 1736 that slides over the barrel1730. One or more keyways (not shown) formed between the inner surfaceof the sleeve 1736 and outer surface of the barrel 1730 can be used torestrict rotation of the sleeve relative to the barrel, while allowingan axial sliding motion (axial being parallel to the optical axis OA.The sleeve 1736 is retained in a desired position by one or more setscrews 1760. The threaded back end 1734 of the barrel can define astandard C-mount size thread in an embodiment. Hence the outer diameterof the barrel 1730 is approximately 25 millimeters. The depicted lenscan handle at least 3-Megapixel resolution in the presence of moderatedrift.

IV. Conclusion

It should be clear that the above-described embodiments, provide asystem that is particularly useful for imaging a small feature (orfeature set), such as an ID code, over a relatively large distance. Theeffect of the variable lens assembly is weakened using the positive lensassembly according to an embodiment. This arrangement is acceptablewithin the desired operational range and feature size. In furtherembodiments, the (e.g. removable) lens arrangement places the variablelens behind the fixed optical components, which generate the reduceddrift characteristic. The variable lens, thus provides the rearmostoptical component of the arrangement before the sensor. The variablelens can be included in the lens arrangement/housing, or can be part ofthe camera assembly.

The foregoing has been a detailed description of illustrativeembodiments of the invention. Various modifications and additions can bemade without departing from the spirit and scope of this invention.Features of each of the various embodiments described above may becombined with features of other described embodiments as appropriate inorder to provide a multiplicity of feature combinations in associatednew embodiments. Furthermore, while the foregoing describes a number ofseparate embodiments of the apparatus and method of the presentinvention, what has been described herein is merely illustrative of theapplication of the principles of the present invention. For example, asused herein various directional and orientational terms such as“vertical”, “horizontal”, “up”, “down”, “bottom”, “top”, “side”,“front”, “rear”, “left”, “right”, and the like, are used only asrelative conventions and not as absolute orientations with respect to afixed coordinate system, such as gravity. Also, while the depicted lensassembly is incorporated in a removable lens unit, it is contemplatedthat the system can be employed in a fixed and/or permanently mountedlens. Likewise, while the above-described lens sizes and spacingdistances are employed for the exemplary operational range, such sizesand distances can be scaled upwardly or downwardly in arrangements thathave similar relative parameters but a larger or smaller overall size.Additionally, where a “lens assembly” is employed and/or describedherein, it can consist of one or more discrete lenses that provide adesired optical effect. Accordingly, this description is meant to betaken only by way of example, and not to otherwise limit the scope ofthis invention.

What is claimed is:
 1. A vision system that compensates for driftcomprising: an image sensor operatively connected to a vision systemprocessor; a variable lens assembly that varies a shape or a refractiveindex thereof; and a fixed lens assembly configured to weaken an effectof the variable lens assembly over a predetermined operational range ofthe object.
 2. The vision system a set forth in claim 1 wherein thevariable lens assembly comprises a liquid lens assembly.
 3. The visionsystem as set forth in claim 2 wherein the liquid lens assembly isvariable over approximately 20 diopter.
 4. The vision system as setforth in claim 1 wherein the fixed lens assembly defines a positiveoptical power.
 5. The vision system as set forth in claim 1 wherein thefixed lens assembly and the variable lens assembly are housed in aremovable lens barrel with respect to a camera assembly body and theimage sensor, the image sensor being located within the camera assemblybody.
 6. The vision system as set forth in claim 5 wherein the cameraassembly body is electrically connected to the variable lens assembly,to provide at least one of power and control thereof, by at least one ofcontact pads and a cable assembly.
 7. The vision system as set forth inclaim 1 wherein the fixed lens assembly comprises one of: (a) a frontlens with a front concave surface and a rear convex surface and acentral biconvex lens spaced from the front lens, (b) a front biconvexlens and a rear stacked lens assembly with a front positive lens, centerbiconcave lens and rear positive lens, (c) a front planoconcave lens anda negative lens, a central stacked lens assembly with a biconvex lensand a planoconvex lens, and a rear biconvex lens and positive lens, (d)a front planoconvex lens and positive lens and a rear positive lens andnegative lens, and (e) a front stacked lens assembly with a biconvexlens and biconcave lens and a rear planoconvex lens and negative lens.8. The vision system as set forth in claim 7 at least one lens of thefixed lens assembly comprises a polymer material.
 9. The vision systemas set forth in claim 7 wherein the fixed lens assembly defines aneffective usable focal range of between approximately 0.3 to 8 meters.10. The vision system as set forth in claim 1 wherein the variable lensassembly is located adjacent to a focal point of the fixed lensassembly.
 11. The vision system as set forth in claim 9 wherein thefocal point is one of either a front focal point or a back focal pointof the fixed lens assembly.
 12. The vision system as set forth in claim1 wherein the fixed lens assembly comprises a front lens assembly and arear lens assembly with the variable lens assembly positionedtherebetween.
 13. The vision system as set forth in claim 12 wherein therear lens assembly defines a positive optical power.
 14. The visionsystem as set forth in claim 12 wherein the front lens assembly has apair of lenses each having convex front surfaces and concave rearsurfaces and a lens having opposing concave surfaces, and the rear lensassembly has a lens having opposing convex surfaces.
 15. The visionsystem as set forth in claim 1 wherein the fixed lens assembly and thevariable lens assembly are part of an overall lens assembly focusinglight on the image sensor and an optical power of the fixed lensassembly predominantly defines an overall optical power of the overalllens assembly.
 16. A variable lens system for a vision system having animage sensor that transmits image data to a processor comprising: avariable lens assembly; and a fixed lens assembly having a focal point,the variable lens assembly being located adjacent to the focal point,wherein the fixed lens assembly and the variable lens assembly are partof an overall lens assembly focusing light on the image sensor and anoptical power of the positive lens assembly predominantly defining anoverall optical power of the overall lens assembly.
 17. The lens systema set forth in claim 16 wherein the variable lens assembly comprises aliquid lens assembly.
 18. The lens system as set forth in claim 17wherein the liquid lens assembly is variable over approximately 20diopter.
 19. The lens system as set forth in claim 16 wherein the fixedlens and the variable lens assembly are housed in a removable lensbarrel with respect to a camera assembly body and the image sensor, theimage sensor being located within the camera assembly body.
 20. The lenssystem as set forth in claim 19 wherein the camera assembly body iselectrically connected to the variable lens assembly, to provide atleast one of power and control thereof, by at least one of contact padsand a cable assembly.